1. Field of the Invention
The present invention relates to optical communication equipment and, more specifically but not exclusively, to optical signal-transmission techniques.
2. Description of the Related Art
This section introduces aspects that may help facilitate a better understanding of the invention(s). Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
Optical fibers that are typically used in optical communication systems have a relatively low threshold for nonlinear optical effects. This characteristic of optical fibers becomes especially pronounced with wavelength-division-multiplexing (WDM) transmission formats, where many closely spaced wavelength channels populate the same optical fiber. As a result, the total light intensity in the fiber core becomes relatively high, which facilitates inter-channel nonlinear optical effects. As the modulation speed in each wavelength channel increases, the optical power needed for each channel tends to increase, which tends to promote intra-channel nonlinear optical effects in addition to the inter-channel effects.
One prior-art approach to dealing with adverse nonlinear effects in the optical fiber is to attempt to suppress all nonlinear effects by placing a relatively low limit on the optical power in the fiber. For example, a representative commercial 128-channel WDM system operating at a bit rate of 10 Gb/s per channel may limit the optical power to approximately −5 dBm/channel, for a total optical power of about 16 dBm. However, a relatively low channel power may result in a relatively low optical signal-to-noise ratio, which disadvantageously increases the bit-error rate (BER). The low optical signal-to-noise ratio may also limit the achievable spectral efficiency per channel and thus the total transmission capacity of the communication system. In addition, optical nonlinearities can be very useful for some applications, such as distributed in-fiber amplification, wavelength conversion, optical-pulse regeneration, dispersion compensation, and optical-signal monitoring. Hence, a more-balanced approach is needed that, on the one hand, enables mitigation of the adverse nonlinear effects and, on the other hand, takes advantage of the useful ones.